package panda.io;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.Reader;
import java.util.Iterator;
import java.util.NoSuchElementException;

import panda.lang.Exceptions;

/**
 * An Iterator over the lines in a <code>Reader</code>.
 * <p>
 * <code>LineIterator</code> holds a reference to an open <code>Reader</code>. When you have
 * finished with the iterator you should close the reader to free internal resources. This can be
 * done by closing the reader directly, or by calling the {@link #close()} or
 * {@link #safeClose(LineIterator)} method on the iterator.
 * <p>
 * The recommended usage pattern is:
 * 
 * <pre>
 * LineIterator it = Streams.lineIterator(file, &quot;UTF-8&quot;);
 * try {
 * 	while (it.hasNext()) {
 * 		String line = it.nextLine();
 * 		// do something with line
 * 	}
 * }
 * finally {
 * 	it.close();
 * }
 * </pre>
 * 
 * @author yf.frank.wang@gmail.com
 */
public class LineIterator implements Iterator<String> {

	/** The reader that is being read. */
	private final BufferedReader bufferedReader;
	/** The current line. */
	private String cachedLine;
	/** A flag indicating if the iterator has been fully read. */
	private boolean finished = false;

	/**
	 * Constructs an iterator of the lines for a <code>Reader</code>.
	 * 
	 * @param reader the <code>Reader</code> to read from, not null
	 * @throws IllegalArgumentException if the reader is null
	 */
	public LineIterator(final Reader reader) throws IllegalArgumentException {
		if (reader == null) {
			throw new IllegalArgumentException("Reader must not be null");
		}
		if (reader instanceof BufferedReader) {
			bufferedReader = (BufferedReader)reader;
		}
		else {
			bufferedReader = new BufferedReader(reader);
		}
	}

	// -----------------------------------------------------------------------
	/**
	 * Indicates whether the <code>Reader</code> has more lines. If there is an
	 * <code>IOException</code> then {@link #close()} will be called on this instance.
	 * 
	 * @return <code>true</code> if the Reader has more lines
	 * @throws IllegalStateException if an IO exception occurs
	 */
	public boolean hasNext() {
		if (cachedLine != null) {
			return true;
		}
		else if (finished) {
			return false;
		}
		else {
			try {
				while (true) {
					String line = bufferedReader.readLine();
					if (line == null) {
						finished = true;
						return false;
					}
					else if (isValidLine(line)) {
						cachedLine = line;
						return true;
					}
				}
			}
			catch (IOException ioe) {
				close();
				throw new IllegalStateException(ioe);
			}
		}
	}

	/**
	 * Overridable method to validate each line that is returned. This implementation always returns
	 * true.
	 * 
	 * @param line the line that is to be validated
	 * @return true if valid, false to remove from the iterator
	 */
	protected boolean isValidLine(String line) {
		return true;
	}

	/**
	 * Returns the next line in the wrapped <code>Reader</code>.
	 * 
	 * @return the next line from the input
	 * @throws NoSuchElementException if there is no line to return
	 */
	public String next() {
		return nextLine();
	}

	/**
	 * Returns the next line in the wrapped <code>Reader</code>.
	 * 
	 * @return the next line from the input
	 * @throws NoSuchElementException if there is no line to return
	 */
	public String nextLine() {
		if (!hasNext()) {
			throw new NoSuchElementException("No more lines");
		}
		String currentLine = cachedLine;
		cachedLine = null;
		return currentLine;
	}

	/**
	 * Closes the underlying <code>Reader</code> quietly. This method is useful if you only want to
	 * process the first few lines of a larger file. If you do not close the iterator then the
	 * <code>Reader</code> remains open. This method can safely be called multiple times.
	 */
	public void close() {
		finished = true;
		Streams.safeClose(bufferedReader);
		cachedLine = null;
	}

	/**
	 * Unsupported.
	 * 
	 * @throws UnsupportedOperationException always
	 */
	public void remove() {
		throw Exceptions.unsupported("Remove unsupported on LineIterator");
	}

	// -----------------------------------------------------------------------
	/**
	 * Closes the iterator, handling null and ignoring exceptions.
	 * 
	 * @param iterator the iterator to close
	 */
	public static void safeClose(LineIterator iterator) {
		if (iterator != null) {
			iterator.close();
		}
	}

}
